Allow parallel tests with isolated environment.

See https://github.com/JuliaLang/julia/issues/43205 and
https://github.com/JuliaLang/julia/pull/43211.

diff --git a/test/runtests.jl b/test/runtests.jl
index 2f9cd058bb..150395e78c 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -4,7 +4,7 @@ using Test
 using Distributed
 using Dates
 import REPL
-using Printf: @sprintf
+using Printf: @sprintf, @printf
 using Base: Experimental
 
 include("choosetests.jl")
@@ -83,11 +83,12 @@ prepend!(tests, linalg_tests)
 import LinearAlgebra
 cd(@__DIR__) do
     n = 1
-    if net_on
+    if net_on || haskey(ENV, "JULIA_CPU_THREADS")
         n = min(Sys.CPU_THREADS, length(tests))
         n > 1 && addprocs_with_testenv(n)
         LinearAlgebra.BLAS.set_num_threads(1)
     end
+    @printf("Number of threads: %i\n", n)
     skipped = 0
 
     @everywhere include("testdefs.jl")
